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The use of lime-sulphur on bonsai to whiten the deadwood is well-known. The lime-sulphur is applied to the wood with a paint-brush and after a few hours to a few days, depending on the condition of the wood itself as well as ambient temperatures, the wood turns white.
There are drawbacks with the use of lime-sulphur though. Recently created deadwood that is still sappy absorbs the lime-sulphur poorly and doesn't whiten well. The tone of the 'white' colour achieved will frequently be a creamy-white that although adequate, is not a more desirable grey/silver white. Lime-sulphur also tends to produce a very oblique colour with little or no variation. Finally, the white that lime-sulphur produces is more often than not, unsuitable for deciduous or broadleaf bonsai that will demand softer, greyer tones.
Newly-styled-yamadori Juniper sabina bonsai with lime-sulphur and ash applied to the deadwood to produce a grey-white rather than a creamy-white colour.
Lime-sulphur and Ash
In the past I have used black ink or paint or indeed ash, mixed into the lime-sulphur itself before application. This has achieved a greyer tone in the deadwood, but looked unnatural as the grey colour is uniform across the deadwood, there is no natural variation that you would expect to see in natural deadwood.
However, this past year I have started experimenting with more natural tones by rubbing slightly-damp ash into the deadwood before applying the lime-sulphur, and I am very pleased with the results!
Newly-carved deadwood on a large Elm bonsai after rubbing in ash and applying lime-sulphur. Notice not only the more natural grey colour of the lime-sulphur, but the variation in greys produced by the ash.
On dry wood I moisten some ash, I use cigarette-ash but burnt newspaper with plenty of newsprint on it works well, and use a toothbrush to rub it into the wood. You need the ash to be just wet enough to moisten the wood but not so much it is truly wet. This way the ash takes in some places and not others. Do not use too much water in the ash or it will produce too uniform a grey. If you rub the ash across the grain, it darkens the grain better.
Then apply lime sulphur, preferably before the ash dries. The damp ash seems to act as a carrier and fixes the lime sulphur really well and lightens the wood lightens very quickly. I have found that this is a really effective way of getting lime sulphur to take on newly created, sappy deadwood.
For darker-grey tones on deciduous or broad-leaf bonsai, apply extra ash and just one application of lime-sulphur. For lighter tones, use a smaller amount of ash and several applications of lime-sulphur.
The freshly exposed wood of a newly carved Olive trunk, typically this wouldn't lime-sulphur white very well and would produce a yellow/cream white.
However, by rubbing ash into the wood first (after carving had been completed), the wood turned a grey-white immediately after applying the lime-sulphur. Note that the deeper areas of the deadwood have been stained with a water-based black paint to produce an even darker colour and contrast.